Can I sue my deceased husband's partner for not buying Estate out (as per buyout agreement?

My deceased husband had a partner(his brother),with whom they had buyout agrement (50x50%) in case one of partners dies,surviving partner need to buyout Estate of deceased partner. It's 2,5 years since my husband died. My husband had a debt that now increased double( from $100,000 to $200,000) ,because of stop payments. Estate can't pay to debtor bcs partner don't want to buy us out and we also facing annual interest fee for this debt. Can I sue that partner for violating (valid signed) agreement and also can he be responsible and pay the debt difference+it's interest? I have a lawyer,who telling me that "It has to be a valid cause of action" in order to sue,but what more valid it could be? Or whatever is happening it's not valid enough? Thank you